How did William Harvey show that blood circulates through the body?

William Harvey demonstrated that blood circulates through the body by using precise observation and experimental evidence, which he detailed in his 1628 work, Anatomical Exercise on the Motion of the Heart and Blood in Animals.12

His discovery relied on several key arguments:

• Quantitative Analysis: Harvey estimated the volume of blood expelled by the heart per beat and the heart's rate of contraction. He showed that the heart pumps more blood than the body could possibly produce or consume, necessitating a closed, circulating system.1
• Venous Valves: He examined the small flaps inside veins, demonstrating that they are oriented to allow blood flow only toward the heart. By showing these valves inhibit flow away from the heart, he disproved the idea that they merely prevented blood from pooling due to gravity.1
• Heart Function: Through observing the heartbeats of various animals, he determined that the active phase of the heartbeat is the contraction (systole), which expels blood with force, rather than a process of drawing blood in.1

While Harvey could not observe capillaries without a microscope, his work established the foundation for modern physiology by prioritizing empirical evidence over traditional authority.12